1. Field of the Invention
The present invention relates to both implantable and external pacemakers and monitors that may be used in an environment of the high static and radio frequency (RF) magnetic field strength generated by magnetic resonance imaging (MRI) systems.
2. Description of the Prior Art
Pacemakers are commonly used to control the heart rate when there is a disorder of the heart rhythm. However, other types of pacemakers, or tissue stimulators, can be used for pain relief, by local nerve stimulation, or for pacing skeletal muscle in handicapped patients. One new type of pacemaker is used to pace both the cardiac and skeletal muscle for patients who undergo cardiomyoplasty. (Cardiomyoplasty is the placement of a skeletal muscle graft on the heart to assist the failing heart.)
Modern pacemakers perform two functions-ECG sensing (input) and cardiac pacing (output). The ECG signal is monitored via one or two electrodes placed on the epicardial or endocardial surface of the heart or the surface of the body. These ECG sensing electrodes are usually connected to a differential amplifier that increases the low level (1 to 10 mV) ECG signal to a higher level (1 V or larger) signal that can be used by the pacing logic. The pacing logic controls pacemaker operation. Depending on the type of pacemaker and the heart chamber from which the electrical activity is sensed, the pacing output amplifier can be either triggered or inhibited. Noise in the ECG signal from any source can interfere with the proper functioning of the pacemaker. The common sources of noise include muscle artifact, electromagnetic field of power lines, and RF noise from electrocautery. Prior art pacemakers are built to exclude this noise.
Magnetic resonance imaging (MRI) is a new and efficient technique used in the diagnosis of many disorders, including neurological and cardiac abnormalities. MRI has achieved prominence in both the research and clinical arenas. It provides a non-invasive method for the examination of internal structure and function. For example, MRI allows one to study the overall function of the heart in three dimensions significantly better than any other imaging method. More recently, MRI imaging with "tagging" permits the non-invasive study of regional ventricular function.
Until now, however, there has been a reluctance to place patients with pacemakers in an MRI apparatus. The environment produced in the MRI apparatus is considered hostile to pacing electronics. The major risk to patients is pacemaker malfunction caused by the electromagnetic fields produced in the MRI system. MRI systems utilize three types of electromagnetic fields: 1) a strong static magnetic field, 2) a time-varying gradient field; and 3) a radiofrequency (RF) field which consists of RF pulses used to produce an image. The static field utilized by current MRI systems has a magnetic induction ranging from 0.5 to 1.5 T. The frequency of the RF field used for imaging is related to the magnitude of the static magnetic field. For the current generation of MRI systems, the frequency of the RF field ranges from 6.4 to 64 MHz. The time-varying gradient field is used in MRI for spatial encoding. The frequency of this field is in the Kilohertz range.
It was originally feared that the static field would create longitudinal forces and torque on the pacemaker case and leads (P. L. Davis, L. Crooks, M. Arakawa, R. McRee, L. Kaufman, A. R. Margulis. Potential hazards in NMR imaging: Heating effects of changing magnetic fields and RF fields on small metallic implants. AJR 137:857-860, 1981.) However, in the case that was reported, no physical damages to the leads, pacemaker or patient could be attributed to the static field. The patient did not complain of mechanical discomfort (B. H. Zimmermann, D. D. Faul. Artifacts and hazards in NMR imaging due to metal implants and cardiac pacemakers. Diagn. Imag. Clin. Med. 53:53-56, 1984.) However, the static magnetic field can affect the magnetically controlled (reed) switch that prevents inappropriate programming of the pacemaker. When the head of the pacemaker programming unit is placed over the pacemaker, the permanent magnet in the programmer head causes the reed switch to close. The pacemaker is then placed in an asynchronous, or safety, pacing mode while programming takes place. When the pacemaker in placed in the MRI, the reed switch is actuated by the static magnetic field forcing the pacemaker to the asynchronous pacing mode (J. A. Erlebacher, P. T. Cahill, F. Pannizzo, R. J. R. Knowles. Effect of magnetic resonance imaging on DDD pacemakers. Am. J. Cardiol. 57:437-440, 1986; J. Fetter, G. Aram, D. R. Holmes, Jr., J. E. Gray, D. L. Hayes. The effect of nuclear magnetic resonance imagers on external and implantable pulse generators. Pace 7:720-727, 1984; D. L. Hayes, D. R. Holmes, Jr., J. E. Gray. Effect of 1.5 tesla nuclear magnetic resonance imaging scanner on implanted permanent pacemaker. JACC, 10:782-786, 1987.) For a pacemaker to work in the MRI environment, it should not contain a reed switch. However, this then requires a new method of programming not taught in the prior art.
It has been thought by many investigators that the time-varying gradient fields do not effect the proper functioning of the pacemaker (J. A. Erlebacher, P. T. Cahill, F. Pannizzo, R. J. R. Knowles. Effect of magnetic resonance imaging on DDD pacemakers. AM. J. Cardiol. 57:437-440, 1986; D. L. Hayes, D. H. Holmes, Jr., J. E. Gray. Effect of 1.5 tesla nuclear magnetic resonance imaging scanner on implanted permanent pacemaker. JACC, 10:782-786, 1987; F. Iberer, E. Justich, W. Stenzl, H. Machler, K. H. Tscheliessnig, J. Kapeller. Nuclear magnetic resonance imaging of a patient with implantable transvenous pacemaker. Herz-Schrittmacher, MZV-EBM Verlage 7:196-199 1987.) However, contrary to what the prior art taught, the present inventors discovered that the time-varying gradient field can generate significant voltage in the ECG leads that may be interpreted by the ECG amplifier as a QRS complex.
The RF field produced in the MRI system represents a form of electromagnetic interference (EMI) that is very hazardous to the pacemaker. The RF pulses can produce two distinct categories of problems: 1) heating, and 2) voltage generation in the pacemaker, its circuitry and leads.
Heating is the result of eddy currents formed in the metal case of the pacemaker. The conductivity of the tissue surrounding the pacemaker can expand the current path to the tissue. No evidence of abnormal heat generation by the pacemaker was reported in patients (F. Iberer, E. Justich, W. Stenzl, H. Machler, K. H. Tscheliessnig, J. Kapeller. Nuclear magnetic resonance imaging of a patient with implantable transvenous pacemaker. Herz-Schrittmacher, MZV-EBM Verlage 7:196-199 1987.) However, new techniques, such as "tagging", that requires increased number of RF pulses may result in increased heat production.
Voltage generated by the RF pulses has been implicated in two general types of pacemaker malfunction: 1) inhibition of pacing; and, 2) excessively rapid pacing. Both of these malfunctions can result in a life-threatening reduction in blood pressure (D. L. Hayes, D. H. Holmes, Jr., J. E. Gray. Effect of 1.5 tesla nuclear magnetic resonance imaging scanner on implanted permanent pacemaker. JACC, 10:782-786, 1987; J. A. Erlebacher, P. T. Cahill, F. Pannizzo, R. J. R. Knowles. Effect of magnetic resonance imaging on DDD pacemakers. AM. J. Cardiol. 57:437-440, 1986; J. Fetter, G. Aram, D. R. Holmes, Jr., J. E. Gray, D. L. Hayes. The effect of nuclear magnetic resonance imagers on external and implantable pulse generators. Pace 7:720-727, 1984; B. H. Zimmermann, D. D. Faul. Artifacts and hazards in NMR imaging due to metal implants and cardiac pacemakers. Diag. Imag. Clin. Med. 53:53-56, 1984.) As previously noted, a pacemaker placed in a static magnetic field reverts to an asynchronous pacing mode. It has been observed, however, that RF pulses generated by the MRI interfere with this safety mode by totally inhibiting the output of the pacemaker. This inhibition of pacing is especially of concern in those patients totally pacemaker dependent. It has also been observed that the RF pulses produced by the MRI system can pace the heart at rates of up to 800/min (J. A. Erlebacher, P. T. Cahill, F. Pannizzo, R. J. R. Knowles. Effect of magnetic resonance imaging on DDD pacemakers. AM. J. Cardiol. 57:437-440, 1986; D. L. Hayes, D. H. Holmes, Jr., J. E. Gray. Effect of 1.5 tesla nuclear magnetic resonance imaging scanner on implanted permanent pacemakers. JACC, 10:782-786, 1987).
Each of the above pacemaker malfunctions is caused by the generation of unwanted voltages in the pacemaker. These unwanted voltages are generated in the pacemaker in the following manner. The leads, electrodes and tissue between electrodes comprise a winding in which the RF field generates electromotive force (EMF). In an MRI system operating at 6.4 Mhz, voltages of up to 20 V peak-to-peak are generated. In unipolar pacemakers where the case acts as the second electrode, the tissue between the intracardiac electrode and the pacemaker case serve as the second lead providing a winding with a large effective area. Even higher unwanted voltages can be detected in such unipolar pacemakers. The EMF generated in the leads by the MRI system is proportional to the frequency of the RF. At the higher RF frequencies expected for the next generation of MRI systems voltages approaching 100 V may be expected.
Pacemakers as taught in the prior art have some EMI/RF protection (B. H. Zimmermann, D. D. Faul. Artifacts and hazards in NMR imaging due to metal implants and cardiac pacemakers. Diag. Imag. Clin. Med. 53:53-56, 1984). It is clear, however, that this filtering is insufficient. When placed in the MRI environment, these prior art pacemakers fail by the following mechanisms:
a) RF pulses propagate along the pacing leads and are delivered directly to the input and output circuitry of the pacemaker. The RF is transmitted directly via the leads into the pacemaker case itself. Once the RF is inside the case, this voltage can propagate along the pacemaker circuitry causing many different types of malfunction, including inhibition or improper pacing. PA0 b) Second, once the RF enters the pacemaker, the pacemaker circuitry may act as a rectifier and demodulator of the EMF. The demodulated signal has a shape similar to a pacing spike and is of significant voltage. Since the output impedance of the output circuitry is low, high current can be produced through the pacing lead. This can result in pacing of the heart with each RF pulse.
The frequencies of RF used for MRI systems require more rigorous methods of noise protection than taught in the prior art. There are two general methods of filtering RF noise--passive and active. Active filters use an operational amplifier and require external power. This method, however, may not be suitable as the primary means of filtering RF. The high voltages generated at the input terminals of the filter by MRI systems (25-100 V as compared to the ECG signal of a few millivolts) can cause the amplifier circuitry to become saturated and severely degrade its performance. On the other hand, passive filters are attractive because they can operate without being saturated by the RF field. One common method of passive filtering in ECG monitors is the use of high resistance to limit current generated by the RF field (U.S. Pat. No. 4,280,507 issued to Rosenberg on Jul. 28, 1981 and U.S. Pat. No. 4,951,672 issued to Buchwald, et al. on Aug. 28, 1990). However, this method is not usable in pacing leads because it limits battery life.
Filters using inductances and capacitances have been previously used in ECG monitors (U.S. Pat. No. 4,245,649 issued to Schmidt-Andersen on Jan. 2, 1981) and pacemakers (U.S. Pat. No. 3,968,802 issued to Ballis on Jul. 13, 1976). These filters were used to protect the monitor and/or pacemaker from RF fields generated during electrocautery, but would not protect the pacemaker in the MRI environment. The circuit elements used in these designs may not function properly at the higher frequencies used by MRIs as the elements appear to have unsuitable frequency characteristics. Further, the inductors used had iron cores (U.S. Pat. No. 4,245,649 issued to Schmidt-Andersen on Jan. 2, 1981). If used in an MRI system, these inductors can be saturated by the high static magnetic field of the MRI, thus lowering their inductance and making their characteristics non-linear. Also, if the ferromagnetic elements are positioned close to the volume being imaged, these elements distort the magnetic field, thus degrading the quality of image and even making the imaging impossible. In addition, the prior art filters, as taught in the above patents, do not filter the lead connected to the ECG reference electrode as well as the shield of the ECG leads and will, therefore, draw the RF noise into the amplifier. Therefore, these prior art filters will not function in the MRI environment.
U.S. Pat. No. 4,887,760 issued to Cole on Dec. 19, 1989 describes a method of filtering an ECG monitor specifically for use in the MRI environment. The filter contains an active element that provides cutoff switching from 50 Hz to 5 Hz when the MRI system is activated. When activated, this filter blocks the RF field, but will also totally block the ECG signal while the MRI imaging sequence is in progress. Since this lockout of the ECG input can last up to two seconds, the monitor could fail to detect a QRS complex for one or more cycles. Though improving the quality of MRI imaging, this method more than doubles the total imaging time. More importantly, the method has limited utility for pacemakers because inhibition of ECG sensing can lead to life-threatening complications.